The present invention relates to a process for purifying acrylic acid by crystallization by means of vacuum evaporation.
Crystallization processes for purifying acrylic acid are known. In crystallization processes based on the generation of supersaturation required for solids formation, a distinction is generally made between cooling crystallization processes and evaporative crystallization processes. Whereas, in cooling crystallization, solids formation is effected by cooling the solution to be crystallized, in evaporative crystallization the solubility limit of the solid to be crystallized in the solution is exceeded by evaporating one or more components of the solution (solvent withdrawal). If the evaporation is carried out under reduced pressure (vacuum evaporation), the crystallizing solution can be additionally cooled by the reduced pressure applied. Crystallization by means of evaporative cooling, in which no heat is supplied and the crystallization is thus carried out as an adiabatic vacuum evaporative crystallization, is to be regarded as a special case of crystallization by means of vacuum evaporation. The vapor leaving the crystallization apparatus in the vacuum evaporative crystallization is usually condensed in a condensation stage. In an evaporative cooling crystallization, some or all of the condensed vapor is usually recycled to the crystallization apparatus (reflux of the condensate into the crystallization). Evaporative cooling as a vacuum evaporation process is therefore situated between the pure cooling process and the pure evaporation process (without reflux of the condensed vapor).
Compared with a cooling crystallization process operating with indirect cooling by means of heat exchangers, the evaporative cooling process has a major advantage of direct cooling of the solution to be crystallized (direct withdrawal of the heat of vaporization from the solution to be crystallized). Operating problems due to incrustations, as may occur in the heat exchangers in indirect cooling, are thus avoided.
The Laid-Open Japanese Patent Application JP 07 082 210-A discloses a process for purifying acrylic acid by crystallization by means of evaporative cooling. In this process, the starting material used is a crude acid which contains little or no water, and water is added to it in order to bring the water content of the mother liquor present during the crystallization to a range of from 2 to 10% by weight. Parts of the acrylic acid/water mixture are evaporated in order to induce adiabatic cooling under reduced pressure, with the result that acrylic acid crystals are precipitated, which are then isolated. The vapor formed during evaporation is passed into a condenser, acrylic acid flowing over the surface of the condenser in order to prevent ice formation, or water flowing over the surface of the condenser in order to prevent precipitation of acrylic acid crystals. The condensate is recycled to the crystallization.
In the light of this prior art, it is an object of the present invention to provide a process which permits the purification of acrylic acid from a crude acid having a relatively high water content, solids formation in the condenser being avoided.
We have found that this object is achieved by crystallizing the acrylic acid from a water-containing solution by means of vacuum evaporation with formation of a liquid phase, consisting of an acrylic acid-containing solution containing more than 10% by weight of water (mother liquor) and the crystals, and a vapor phase, introducing the vapor phase into a liquid material in a condensation zone with the formation of a liquid condensation mixture and recycling the condensation mixture at least partly to the condensation zone.
The present invention therefore relates to a process for purifying acrylic acid by crystallization by means of vacuum evaporation, wherein a solution which contains acrylic acid and water and may contain further components is crystallized by means of vacuum evaporation with formation of a liquid phase, consisting of a mother liquor containing more than 10% by weight of water and of the crystals, and a vapor phase, the vapor phase is introduced into a liquid material in a condensation zone with formation of a liquid condensation mixture or condenser mixture, and the liquid condensation mixture is recycled at least partly to the condensation zone, the operating conditions in the condensation zone being established so that no solid is precipitated therein.
Preferred embodiments of the invention are described in the subclaims, in the following description of the Figures and in the Example. Here, the terms xe2x80x9ccondenser mixturexe2x80x9d and xe2x80x9ccondensation mixturexe2x80x9d have the same meaning. The terms xe2x80x9clow-boiling componentsxe2x80x9d and xe2x80x9cmedium-boiling componentsxe2x80x9d relate to components which have, respectively, a lower boiling point than acrylic acid and about the same boiling point as acrylic acid.